Bring back the `INSERTKEY_XYZ_TO_RGB` enum item for the
`keyframe_insert()` function (it was removed in 30b0c5b225). This way
any Python code that targets Blender 4.x can safely pass this flag,
without having to check specific Blender versions.
Note that the flag is implemented as a no-op, as the behaviour change
introduced in 30b0c5b225 (just looking at the user preference) is still
retained. The purpose of this commit is simply to avoid the `ValueError`
exception that would otherwise be raised.
This should also fix Rigify report blender/blender-addons#105241.
Sybren A. Stüvel